The present invention relates to fuel cell based concentration sensors and extending the measuring range thereof. Further, it relates to methanol concentration sensors suitable for use in direct methanol fuel cell systems.
Fuel cells have been used as sensors for measuring the concentration of various oxidizable species in gas or liquid mixtures. An important application has been in the determination of methanol content in breath samples or in the headspace above beer and/or wine during fermentation processes. Generally, the operation of fuel cell based sensors involves supplying the fluid mixture to be analyzed to the fuel cell anode and then measuring the electrical output of the fuel cell. No external potential or current is supplied to the sensor and the measured electrical output is typically proportional to the concentration of the oxidizable species present in the mixture.
Fuel cell systems have also been used historically as power supplies in certain specialized applications but are receiving increased attention of late for use in more general applications, including power supplies for various portable, motive, and stationary applications. In some fuel cell systems, it is necessary to determine the concentration of oxidizable species in certain fluid mixtures and thus it may be useful to employ a fuel cell sensor as a component in the fuel cell based power supply.
In general, electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A solid polymer fuel cell is a specific type of fuel cell that employs a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. The electrocatalyst used may be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The electrocatalyst is typically incorporated at the electrode/electrolyte interfaces. This can be accomplished, for example, by depositing it on a porous electrically conductive sheet material, or xe2x80x9celectrode substratexe2x80x9d, or on the membrane electrolyte. Flow field plates for directing the reactants across one surface of each electrode substrate are generally disposed on each side of the MEA. Solid polymer fuel cells typically operate in a range from about 40xc2x0 C. to about 150xc2x0 C.
A broad range of reactants has been contemplated for use in solid polymer fuel cells and such reactants may be delivered in gaseous or liquid streams. The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air. The fuel stream may, for example, be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream derived from a suitable feedstock, or a suitable gaseous or liquid organic fuel mixture. Liquid feedstocks and/or fuels, such as methanol, are preferred, particularly in non-stationary applications, since they are relatively easy to store and handle. Where possible, it is advantageous to react a fuel mixture directly in the fuel cell (that is, to supply the fuel unreformed to the fuel cell anodes) in order to avoid using a reformer in the fuel cell system. Inside the fuel cell, the fuel mixture may be reacted electrochemically (directly oxidized) to generate electricity or instead it may be reformed in-situ (internally reformed), as in certain high temperature fuel cells (for example, solid oxide fuel cells).
A direct methanol fuel cell (DMFC) is a type of solid polymer fuel cell that operates directly on a methanol fuel stream typically supplied as a methanol/water vapor or as an aqueous methanol solution in liquid feed DMFCs. The methanol in the fuel stream is directly oxidized at the anode therein. There is often a problem in DMFCs with crossover of methanol fuel from the anode to the cathode side through the membrane electrolyte. The methanol that crosses over typically then reacts with oxidant at the cathode and cannot be recovered, resulting in significant fuel inefficiency and deterioration in fuel cell performance. To reduce crossover, very dilute solutions of methanol (for example, about 5% methanol in water) are typically used as fuel streams in liquid feed DMFCs. The fuel streams in DMFCs are usually recirculated in order to remove carbon dioxide (a by-product of the reaction at the anode) and to re-use the diluent and any unreacted fuel in the depleted fuel stream exiting the DMFC). Methanol is added to the circulating fuel stream before it re-enters the fuel cell in order to compensate for the amount consumed, thereby providing a fresh mixture at the desired methanol concentration. Since the amount of methanol consumed is variable (depending on the load, crossover, and other operating parameters), the methanol concentration in the circulating fuel stream is usually measured continuously with a suitable sensor, and fresh methanol is admitted in accordance with the signal from the sensor.
Various types of sensors may be considered for purposes of measuring the concentration of methanol in aqueous solution and thus for use in a recirculating fuel stream in a liquid feed DMFC. For instance, a fuel cell based sensor may be considered. In Japanese Published Patent Application No. 56-118273, a small capacity, temperature-compensated fuel cell is suggested for use as a concentration sensor for a liquid electrolyte air-methanol fuel cell. A signal from the sensor is obtained by measuring the voltage across a large resistance connected across the small capacity fuel cell. In xe2x80x9cFuel Cell Sensorsxe2x80x9d, Selective Electrode Rev., 1992, Vol. 14, pp. 125-223, W. J. Criddle et al. discuss the principles and applications of fuel cell sensors generally. It was noted that diffusion from the air is usually sufficient for the oxygen supply in fuel cell sensors. Thus in principle, as long as the species concentration to be measured is in an appropriate range, a conventional direct methanol fuel cell may be employed as the concentration sensor. Saturation of such a sensor (where the electrical output of the sensor levels off and the sensor is no longer responsive to an increase in concentration) may occur however at higher methanol concentrations.
A fuel cell based concentration sensor for use in DMFCs is disclosed in U.S. Pat. No. 5,624,538. Therein, an additional, diffusion limiting, membrane is employed against the side of the anode opposite the ion conducting membrane electrolyte in the sensor fuel cell. The diffusion limiting membrane is used to limit the transport of methanol. It was suggested that the measuring range of this sensor could be expanded by varying the thickness of the diffusion limiting membrane.
Other types of sensors include capacitance devices or amperometric devices. The former measure the change in dielectric constant of the fuel stream with methanol concentration. The latter measure the current output from electrochemical cells in which an external potential is applied across the electrochemical cells and current is generated in accordance with the species concentration. The devices described in PCT/International Publication No. WO 98/45694 (Application No. PCT/US98/07244) and J. Electrochem. Soc., Vol. 145, No. 11, Nov. 1998, p. 3783 are examples of the latter. Along with an electrical meter for measuring the output of these devices, an additional apparatus is required to apply an external potential.
A preferred sensor however would measure methanol concentration over an extended range, without saturating at the higher methanol concentrations of interest, and without requiring an external power supply.
The present invention provides for extending the measuring range of a fuel cell based sensor that measures the concentration of an oxidizable species in a gaseous or liquid fluid mixture. This is accomplished by decreasing the load applied across the fuel cell terminals and by increasing the oxidant supply such that saturation of the sensor fuel cell electrical output is avoided over the extended measuring range. A sensor with such an extended range is particularly suited for use in DMFC systems because, in many circumstances, the construction and/or operating conditions of conventional DMFCs are such that they are not capable of measuring methanol concentration over a satisfactory range for the purposes of the DMFC system. In conventional direct methanol fuel cell types, the electrical output of the sensor can become saturated at high methanol concentrations, thereby rendering the sensor ineffective in that range.
The concentration sensor comprises a solid polymer fuel cell in which the oxidizable species can be directly oxidized, a load across the terminals of the fuel cell, and an electrical meter for measuring an electrical output of the fuel cell. The fuel cell comprises an anode electrode supplied with the fluid mixture comprising the oxidizable species, a cathode electrode supplied with an oxidant at an oxidant stoichiometry, and a solid polymer electrolyte. (Oxidant stoichiometry is defined as the ratio of the rate oxidant is supplied to the cathode to that consumed in the electricity generating reaction of the fuel cell.) Any oxidant that reacts with crossed over species at the cathode is not included in the amount consumed since no useful external electricity is generated.) The oxidizable species may be an alcohol (for example, methanol), an ether (for example, dimethyl ether), an aldehyde, or an ester. In particular, the fluid mixture may be a liquid aqueous methanol solution and the fuel cell may be a direct methanol fuel cell. The operating temperature of the fuel cell may preferably be between about 40xc2x0 C. and 100xc2x0 C.
To extend the measuring range of the sensor, the load across the fuel cell terminals is decreased thereby increasing the current density in the fuel cell. The electrochemical consumption of the oxidizable species at the anode is thus increased, which in turn can result in a diffusion limited situation for the species within the anode, thereby avoiding saturation. Additionally, less of the species is available to crossover the electrolyte membrane to the cathode and thus adverse effects due to crossover are reduced. (Once the oxidizable species has crossed over to the cathode, it can compete for and react with the oxidant intended for electrochemical reaction with already oxidized (that is, measured) species. Further, this competing reaction lowers the cathode potential.) In a methanol concentration sensor, an extended measuring range may be obtained by decreasing the load across the fuel cell terminals to less than about 10 ohms. The range can be extended further by decreasing the load across the fuel cell terminals to less than about 10 milliohms.
The oxidant stoichiometry is also increased to suppress oxidant mass transport limitations. A sensor having extended measuring range may operate at an oxidant stoichiometry greater than about 2 and thus would include oxidant supply means capable of supplying the sensor""s fuel cell at that stoichiometry. A conventional apparatus may be used to supply the increased oxidant. Conveniently, if the sensor is used as a component in a power supply comprising a direct oxidation fuel cell stack, the source of the oxidant supplied to the cathode of the sensor""s fuel cell may be the same as that supplied to the direct oxidation fuel cell stack.
With a suitable choice of load and oxidant stoichiometry, the measuring range of a methanol concentration sensor can be extended to measure methanol concentrations from 0 M to 4 M in liquid aqueous methanol solution. The measured electrical output of the fuel cell in the sensor may be essentially proportional to the methanol concentration over the range 0 M to 2 M, thus providing a linear response to concentration. It may be advantageous to provide means for varying the load and/or the rate of oxidant supply during operation of the concentration sensor such that its measuring range may be varied during operation.
The electrical output measured may be either the current through the load or the voltage across the load. Thus, the electrical meter in the sensor may be either an ammeter or a voltmeter.
Preferably, the sensor is relatively small since this provides several advantages including superior response times. For instance, the area of the electrodes in the sensor""s fuel cell may be less than about 1 cm2. A flow field plate for such a miniature fuel cell can be of simple construction, for instance comprising a single flow channel.
A concentration sensor of the invention is particularly suited for use in a power supply comprising a fuel cell stack supplied with a recirculating fuel stream, for example, a DMFC stack. The sensor is used to measure the concentration of the oxidizable species in the recirculating fuel stream and to direct the operation of a device that adjusts the concentration of the oxidizable species in the recirculating fuel stream.